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Derivative Instruments (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Sep. 30, 2009
Derivative Instruments (Textual) [Abstract]      
Notional amount of interest rate swap     $ 750
Fixed interest rate under interest rate swap agreement     8.125%
Spread on variable rate based on one month LIBOR     4.625%
Variable interest rate 4.896% 4.886%  
Reduction in interest expense 24 24  
Other Assets [Member]
     
Derivatives, Fair Value [Line Items]      
Fair value of the interest rate swaps 95 19  
2019 Notes [Member]
     
Debt Instrument [Line Items]      
Adjusted carrying amounts of the hedged debt outstanding $ 845 $ 769